Skripal and daughter Yulia were hospitalized in March after they were attacked in Britain with the Soviet-era nerve agent novichok. Yulia has been released but Sergei Skripal is still recovering.

"The reckless attempted assassination of Sergei Skripal, using a highly lethal nerve agent, put numerous lives at risk, including that of his daughter," Parker said Monday, accusing the Kremlin of "flagrant breaches of international rules."

Britain, the United States and other Western nations say Russia was behind the attack. British and U.S officials expelled Russian diplomats in response, and Russia answered by removing British and U.S. diplomats from Moscow.